The impulse-heat sealing method occurs when the start signal is synchronized with the machine rhythm, every heatsealing cycle is followed by a cooling phase with jaws closed.
Cooling is effected with jaws closed so that the seal has already set and has good strength and appearance when the jaws are opened. This method is preferred when sealing time is not critical since seam strength and appearance are better.
Since the heat sealing is a thermodynamic reversible procedure, and often influenced by time restraints, it is very important that the relevant parameters: temperature, time and pressure are very carefully synchronized with regards to time and temperature cycle.
The following diagram shows an example of the timely setting of temperature and jaw movement.
The preset temperature should always be attained when the sealing jaws are still open. This allows the sealing element to expand without interference and avoids overstressing of the ends (phase 2).
The system should be designed to drive the heatseal band to the preset temperature in minimum time. However, the maximum rate of temperature rise should not exceed ,1°C./mSec. The total system, principally the secondary voltage, must be optimised to achieve the best result. (Our application service will give you the necessary data for your special application.)
The cool down of the heatseal band while still under the force of the jaws is the main advantage of impulse sealing (phase 5). After turning off the energy most of the excess heat is absorbed by the jaws.
Cooling of the jaws is sometimes recommended so that they can absorb enough heat from the heatseal band in less time.
Contrary to the constant heat feature, a constant heat flow between the sealing element and the jaws must be assured by using a thin backup material with good thermal conductivity behind the heatseal band.
Under certain conditions, sealing is also possible with the so-called rest-heat procedure. During such a phase the remaining heat in a sealing element is calculated just to be enough for the sealing process. This means that in the timing process, phase 4 goes to zero in favor of phase 5.
The current is cut off when the jaws touch. The heat flows into the film and "unloads" the heatseal band, which results in a fast cooling phase. For this method to be effective the thickness, therefore the mass, of the heatseal band must be great enough to hold the required total heat.
PHASE 4 .
Occasionally, in spite of the high response of the controller, the first seal at start-up may not be hot enough. This is because there is not enough time for the heatseal element to reach its pre-set temperature from ambient temperature in the time allowed. This can be avoided by pre-heating the heatseal band to a moderate (80...100°C) preheat before starting. This will help compensate for the first cycle. The temperature in subsequent cycles will never return to ambient because of the remaining heat in the heatseal element from the preceding cycle.
Frequently, to obtain optimum speed, the machine must be timed to allow enough time for the seal to heat and then set-up before opening the jaws to release the film material. Automatic machines may operate at speeds that do not allow time for the heatseal band temperature to return to ambient before the next cycle. Therefore, for highest speed one need allow only enough time for the sealing temperature to recover from the residual temperature level that exists at jaw open position.
If one now sets the timing for high speed operation, it is possible that there will not be sufficient time for the heatseal band to reach the set-point on the first cycle. In this situation one can proceed with the expectation that the first one or two products may be sub-standard or alternately implement a preheat cycle.